1. Field of the Invention
The present invention relates generally to high temperature superconductors and more specifically to high temperature superconductors including mercury.
2. Description of the Background Art
The highest known superconducting transition temperature (T.sub.c) at ambient pressure occurs in the system HgBa.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.y. The next highest set of transition temperatures exists in the TlBaCaCuO layered cuprate family. For the former compounds, as many as four Cu--O layers have been made, but are accompanied by only one Hg--O layer in a unit cell. For the latter compounds, the maximum number of Cu--O layers is four, but both single and double Tl--O phases exist and they have higher transition temperatures. Attempts to increase T.sub.c in these compounds by doping have not succeeded: Substituting Sr for Ba significantly reduces T.sub.c in the Tl system and apparently has a similar effect in the Hg system. Recently, there was one report of a Hg/Tl intergrowth forming a unit cell wherein a 1201/Hg was combined with a 2201/Tl, with a T.sub.c about half that of either end member. Later, the synthesis of Hg.sub.0.5 Tl.sub.0.5 Ba.sub.2 (Ca.sub.1.72 Sr.sub.0.28)Cu.sub.3 O, with a single Hg/Tl layer and T.sub.c values of 128-132 K was reported. Also, a (Hg,Tl).sub.2 Ba.sub.2 CaCu.sub.2 O.sub.y material with a T.sub.c of 100 K, and a (Hg,Tl)Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y were reported.
Hot isostatic pressing (HIPping) has been used to prepared superconducting sample that were difficult to prepare by other means. In addition to offering the element of safety, the use of a hot isostatic press (HIP) allows the operator to select a broader range of reaction temperatures and pressures than are normally accessible in conventional processing. Also, not only to the high pressures during the HIPping process contain the high Hg pressures during HIPping of Hg-containing precursors, but HIPping permits the formation of shields and test bars.